LOS ANGELES (AP) — A spate of incidents in which Los Angeles police officers are accused of using excessive force is threatening to unravel the Police Department's efforts to bolster community relations.

Video in recent weeks has captured officers punching a handcuffed suspect and slamming a restrained woman to the ground. In the third case, a woman stopped breathing in the back of a police car and later died.

The incidents come after years of reforms by the LAPD to improve its officers' conduct.

The efforts, undertaken under a federal court order, have yielded results: The city's crime rate is the lowest in four decades, and police are getting praise from communities, including minorities and gays, that had historically complained about mistreatment.

But the recent incidents underscore that for some police officers on the street, the line on acceptable use of force remains blurry.

"There's been a real effort to change the culture," said Joe Domanick, a professor at the John Jay College of Criminal Justice who is writing a book on the transformation of the LAPD after the Rodney King beating. "It takes a long time for a culture to change, for things to trickle down."

Andrea Ordin, president of the Los Angeles Police Commission, the department's civilian oversight panel, said it's too early to tell if a trend toward overaggressive tactics is developing.

"We are seeing community groups coming to our meetings and talking about the changes in the Police Department and that they appreciate the better communication," she said. "At the same time, we're seeing these incidents. Each of these has to be investigated thoroughly."

In one incident, Alesia Thomas died in a squad car July 22 after struggling with officers trying to arrest her on suspicion of child endangerment. She had left her two children at the police station where they told officers their mother didn't want them, and they had not eaten for several days, said Cmdr. Andrew Smith.

During the struggle to get Thomas into the patrol car, a female officer was captured by the car's video camera threatening to kick Thomas in the groin if she did not obey, and then she made good on the threat. A short while later, Thomas stopped breathing.

Police Chief Charlie Beck has reassigned the five officers involved and ordered an investigation.

On Aug. 21, registered nurse Michelle Jordan was pulled over for holding her cellphone while driving. She got out of the car and did not comply with an officer's demand to get back in the vehicle. The 5-foot-4-inch woman was slammed to the ground by two male officers and handcuffed. While being led to the patrol car, another officer slammed her to the ground again.

The incident was captured by a surveillance camera at a nearby restaurant.

The officers involved were removed from patrol duty pending an investigation, and their supervising captain was transferred. Beck said the videotape raised serious concerns and ordered it shown at officer roll calls throughout the department.

In a third incident, college student Ronald Weekley Jr. was stopped Aug. 18 by officers for riding his skateboard on the wrong side of the street. A bystander's cellphone video appears to show an officer punching the 20-year-old in the face while he was on the ground being handcuffed with four officers on top of him. He said he suffered a broken nose and cheekbone.

Weekley was arrested on suspicion of resisting arrest. Police have launched an internal investigation to determine whether excessive force was used, but the officers involved remain on duty.

"These are unarmed, handcuffed suspects. It's a disturbing, growing trend of police brutality throughout Southern California," said Najee Ali, of the South Los Angeles civil rights group Project Islamic Hope. He noted two other nearby cases that outraged citizens: The police killing of two suspects last month in Anaheim, which sparked a riot, and the July 2011 fatal beating of a handcuffed, homeless, mentally ill man in Fullerton.

Smith, the LAPD commander, said use-of-force incidents are not on the rise. The department records about 1,600 such cases annually, a number that has remained constant in recent years.

"That's out of the millions of contacts we have with the public and the tens of thousands of arrests. Because there's videotape in these incidents, they get a lot of media attention," Smith said.

Officers are permitted to use "reasonable" force to make an arrest, prevent a suspect's escape and overcome resistance. "The question is, is it objectively reasonable? If it isn't reasonable, we hold them accountable," Smith said.

Officers are also extensively trained to verbally persuade people to do what they want without resorting to force, said Richard Weinblatt, a former police chief and police consultant who is a dean at Ivy Tech Community College in Indiana.

Police academy trainers, however, notice that younger cadets, who grew up communicating via text messages and email, and in an era where schoolyard fights are not tolerated as they once were, often lack verbal communication skills and experience in dealing with physical aggression, Weinblatt said. At the same time, criminals are more violent than ever.

"Officers are just plain scared," Weinblatt said. "They don't have the communication skills or physical confidence of previous generations. They're jumping higher and quicker on use of force because of that fear element."